Sam liked Jo right from the off, but hesitated about making any move because she was so obviously more interested in his brother.
When Meg possessed Sam, she noticed his interest in Jo immediately. That was one of the reasons she eventually targeted Jo, after killing a random hunter wasn't enough. If she couldn't force Dean to kill Sam, she was at least going to do her best to hurt them.
After Born Under A Bad Sign, Sam wanted to contact Jo to apologise. He phoned her, but before she could answer he hung up because he doubted she wanted to hear from him then.
